Multiple Possible Destinations Based on Existence

Hey all, thanks in advance for the input. Is there a way to use the Ionics Isapi Rewrite Filter only if the attempted URL has failed? Essentially, I have a scenario where I would like to have a given url, say /products/index.html. Now, ideally, this file
would literally exists in that directory and no rewriting would be needed, no additional application server, etc. However, in cases where /products/index.html does not exist on the file system (and would in turn generate a 404 error), I would THEN like to
use the filter to rewrite the URL.